Wang Changwei, Danovich David, Shaik Sason, Mo Yirong
Department of Chemistry, School of Science, China University of Petroleum (East China) , Changjiangxi Road 66, 266580 Tsingtao, China.
Institute of Chemistry and Lise Meitner Minerva Center for Computational Quantum Chemistry, The Hebrew University , Jerusalem 91904, Israel.
J Chem Theory Comput. 2017 Apr 11;13(4):1626-1637. doi: 10.1021/acs.jctc.6b01133. Epub 2017 Mar 14.
Typical hydrogen and halogen bonds exhibit red-shifts of their vibrational frequencies upon the formation of hydrogen and halogen bonding complexes (denoted as D···Y-A, Y = H and X). The finding of blue-shifts in certain complexes is of significant interest, which has led to numerous studies of the origins of the phenomenon. Because charge transfer mixing (i.e., hyperconjugation in bonding systems) has been regarded as one of the key forces, it would be illuminating to compare the structures and vibrational frequencies in bonding complexes with the charge transfer effect "turned on" and "turned off". Turning off the charge transfer mixing can be achieved by employing the block-localized wave function (BLW) method, which is an ab initio valence bond (VB) method. Further, with the BLW method, the overall stability gained in the formation of a complex can be analyzed in terms of a few physically meaningful terms. Thus, the BLW method provides a unified and physically lucid way to explore the nature of red- and blue-shifting phenomena in both hydrogen and halogen bonding complexes. In this study, a direct correlation between the total stability and the variation of the Y-A bond length is established based on our BLW computations, and the consistent roles of all energy components are clarified. The n(D) → σ*(Y-A) electron transfer stretches the Y-A bond, while the polarization due to the approach of interacting moieties reduces the HOMO-LUMO gap and results in a stronger orbital mixing within the YA monomer. As a consequence, both the charge transfer and polarization stabilize bonding systems with the Y-A bond stretched and red-shift the vibrational frequency of the Y-A bond. Notably, the energy of the frozen wave function is the only energy component which prefers the shrinking of the Y-A bond and thus is responsible for the associated blue-shifting. The total variations of the Y-A bond length and the corresponding stretching vibrational frequency are thus determined by the competition between the frozen-energy term and the sum of polarization and charge transfer energy terms. Because the frozen energy is composed of electrostatic and Pauli exchange interactions and frequency shifting is a long-range phenomenon, we conclude that long-range electrostatic interaction is the driving force behind the frozen energy term.
典型的氢键和卤键在形成氢键和卤键复合物（表示为D···Y-A，Y = H和X）时，其振动频率会发生红移。在某些复合物中发现蓝移现象引起了极大的关注，这引发了对该现象起源的大量研究。由于电荷转移混合（即键合体系中的超共轭）被视为关键作用力之一，比较电荷转移效应“开启”和“关闭”时键合复合物的结构和振动频率将具有启发性。通过采用块定域波函数（BLW）方法可以实现关闭电荷转移混合，该方法是一种从头算价键（VB）方法。此外，利用BLW方法，可以从几个具有物理意义的方面分析复合物形成过程中获得的整体稳定性。因此，BLW方法为探索氢键和卤键复合物中红移和蓝移现象的本质提供了一种统一且物理上清晰的方式。在本研究中，基于我们的BLW计算，建立了总稳定性与Y-A键长变化之间的直接关联，并阐明了所有能量成分的一致作用。n(D) → σ*(Y-A)电子转移使Y-A键伸长，而相互作用部分靠近引起的极化减小了HOMO-LUMO能隙，并导致YA单体内部的轨道混合更强。因此，电荷转移和极化都使Y-A键伸长的键合体系稳定，并使Y-A键的振动频率发生红移。值得注意的是，冻结波函数的能量是唯一倾向于Y-A键收缩的能量成分，因此是相关蓝移的原因。Y-A键长的总变化和相应的伸缩振动频率因此由冻结能量项与极化和电荷转移能量项之和之间的竞争决定。由于冻结能量由静电和泡利交换相互作用组成，且频率移动是一种长程现象，我们得出结论，长程静电相互作用是冻结能量项背后的驱动力。
